In general, TFT liquid crystal panels are constructed by filling liquid crystals between an array side substrate having TFT devices built therein and a color filter-bearing substrate. They are based on the active matrix addressing scheme where TFTs apply controlled voltages for controlled alignment of liquid crystals.
So far, liquid crystal panels have progressed to higher definitions from VGA to SVGA, XGA, SXGA, UXGA and QXGA. It is believed that degrees of definition ranging from 100 pixels per inch (ppi) class to 200 ppi class are necessary. This, combined with an expanding exposure range, imposes a strict exposure accuracy, especially overlay accuracy.
In the manufacture of the array side substrate, patterns are formed in plural layers on a mother glass such as non-alkaline glass by repeating light exposure through originals having circuit patterns drawn thereon, known as large-size photomasks. On the other hand, the color filter side substrate is manufactured by a lithographic process known as dye immersion process. In the manufacture of both array and color filter side structures, large-size photomask substrates are necessary. For a high accuracy of light exposure, such large-size photomask substrates are typically made of synthetic quartz glass characterized by a low coefficient of linear expansion.
Some panels are manufactured using the technology known as low-temperature polysilicon. In this case, it has been studied to bake a driver circuit or the like on a peripheral portion of glass, aside from the panel pixels, which requires light exposure of higher definition.
To accomplish a higher accuracy of light exposure, the flatness of substrates is important. There is a need for large-size photomask-forming substrates exhibiting a higher flatness in service, that is, when supported in an exposure apparatus.
On the other hand, large-size photomask substrates made of synthetic quartz are expensive. Once used as a photomask, the substrate becomes useless or wasteful. A substantial economical benefit would be obtainable if the used substrate were regenerated by baking another mask pattern.
However, in order to reuse large-size photomask substrates, they must be polished again to remove damages and stains which are inadvertently introduced during continuous exposure, transportation, film removal and other operations. Since the glass is thermally affected by image writing, it has thermal strains left within the bulk, which lead to local differences in the polishing rate. As a result, polishing may provide quartz glass with a stepped surface. It is thus important to select polishing conditions so as to eliminate such strains effectively while removing a minimal quantity of material.
The large-size photomask-forming substrate is processed for reuse in such a way that its thickness is reduced whenever it is repolished. As the large-size photomask-forming substrate becomes thinner, it undergoes a more deflection by its own weight at the horizontal attitude. Then substantial variations develop in the proximity gap between the photomask substrate and the motor glass serving as an array side or color filter side substrate in a TFT liquid crystal panel. This eventually reduces the exposure accuracy.
Addressing these problems, the inventors proposed in JP-A 2003-292346 and JP-A 2004-359544 a method for improving the flatness of a large-size glass substrate having a diagonal length of at least 500 mm, achieving a flatness/diagonal length of 4.8×10−5 or less and a parallelism of 50 μm or less in the horizontal attitude.
However, no investigations have been made on the effective recycle of large-size photomask substrates wherein polishing conditions are selected so as to efficiently eliminate the above-mentioned strains and the like while removing a minimal quantity of material.